1. Field of the Invention
The present application relates to an over-current protection device, and more particularly to a surface-mountable over-current protection device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Over-current protection devices are used for protecting circuitries from damage caused by over-heat or over-current. An over-current protection device usually contains two electrodes and a resistive material disposed therebetween. The resistive material has positive temperature coefficient (PTC) characteristic that the resistance thereof remains extremely low at room temperature and instantaneously increases to thousand times when the temperature reaches a critical temperature or the circuit has over-current, so as to suppress over-current and protect the cell or the circuit device. When the resistive material gets back to the room temperature or over-current no longer exists, the over-current protection device returns to be of low resistance and as a consequence the circuitry again operate normally. In view of the reusable property, the PTC over-current protection devices can replace traditional fuses, and have been widely applied to high density circuits.
When a low-resistance and high-current PTC device is surface-mounted to a printed circuit board or a mother board of which the connecting line has different widths according to various designs. Most of the copper connecting lines of the mother board connecting to the PTC device have a larger circuit area, therefore the trip time of the PTC device may be out of the specification. Owing to small difference of abnormal current and hold current of the PTC device and more efficient heat dissipation of the connecting line with a larger circuit area, the heat accumulation rate of the PTC device is lowered and therefore the trip time of the PTC device will be delayed.
If abnormal over-current event is not removed for a long time, the PTC material may be deteriorated, burned, and/or a short circuit may occur between the upper and lower metal foils of the PTC material layer. As a result, the PTC device no longer provides over-current protection and therefore the circuits of the mother board or the circuit boards may be burned or damaged also.